Answer 1

Use a looping...

let valid=false;
do{
let confirm = prompt("Enter passcode to view site");
switch (confirm) {
case "123": alert("Welcome to website");
valid=true;
break;
default: alert("Try again");
valid=false;
break;
}
}while(!valid);

Answer 2

I don't think. Alert makes background program to stop until you press ok. So time setting won't run on alert.
But you can alternatives, of making custum alert through a function or div element and set timeout..

https://stackoverflow.com/questions/15466802/how-can-i-auto-hide-alert-box-after-it-showing-it

https://stackoverflow.com/questions/1962861/javascript-alert-box-with-timer